The need in cycling for efficient, fast shifting from a sprocket of one diameter to another has long been recognized. The vast majority of shifting methods involve a standard derailleur which forces the chain on the holding sprocket toward an adjacent or receiving sprocket until a cog of the adjacent sprocket catches the chain, which causes the chain to become entrained around the sprocket.
These prior derailleurs have proven deficient, however, in that they are incapable of maintaining a positive drive connection throughout the full shifting process. That is, the chain must bridge the gap between the two sprockets while being shifted and thus a full, effective sprocket diameter cannot be maintained. Under normal operating conditions, derailleur shifting mechanisms generally require a reduction in the torque exerted on the sprocket by the chain to allow the chain to jump from one sprocket to another. Therefore, derailleur shifting mechanisms are incapable of shifting the chain to a different sprocket when a force of a relatively high magnitude is being applied to the sprocket.
U.S. Pat. No. 4,127,038 discloses a sprocket assembly having a hinged swinging sector that pivots relative to a stationary sector to align the holding sprocket with the receiving sprocket. The chain is picked up immediately by the receiving sprocket without having to first cross a gap between the holding and receiving sprockets. This type of sprocket assembly effectively provides a full sprocket diameter for the chain to engage throughout the shifting process before the swinging sector pivots back to its normal position.
U.S. Pat. Nos. 4,580,997 and 4,894,046 disclose sprocket selector devices used in conjunction with sprocket assemblies such as the one shown in U.S. Pat. No. 4,127,038. A pawl extending outwardly of the swinging sector passes through a control unit as the sprocket assembly is rotates with the wheel. To shift the chain from one sprocket to another, the control unit deflects the pawl and attached swinging sector in the direction of the receiving sprocket on the stationary sector so that the holding sprocket of the swinging sector becomes aligned with the receiving sprocket and remains in the deflected position until the chain is shifted.
These sprocket shifting assemblies do not involve forcing the chain diagonally across the gap between the holding sprocket and the receiving sprocket, as is done when standard derailleurs are used. A major advantage of the above-mentioned sprocket shifting assemblies is that the rider can shift the chain from one sprocket to another while the sprocket assembly is under a torque of a relatively high magnitude. Another advantage of these sprocket shifting assemblies is that pivoting of the swinging sector (and thus shifting the chain from one sprocket to another) requires a minimal amount of energy.
A few problems have developed, however, in using the above-mentioned sprocket assemblies. First, the close proximity of the control unit to the sprocket assembly creates problems in removing the wheel and rear sprocket assembly from the bicycle frame. The control unit must be mounted on the bicycle frame close enough to the sprocket assembly to ensure that the pawl passes through the control unit. Traditionally, removal of the wheel and sprocket assembly has required complete removal of the control unit from the bicycle frame to allow clearance for the wheel and sprocket assembly.
Second, the control unit must be finely adjusted to accept the pawl without interference in the absence of shifting. The fine adjustments have traditionally been difficult to make because previous mounting arrangements have required that adjustments be made before the control unit is coupled to the bicycle frame. Therefore, if the person adjusting the control unit guesses wrong, or if the control unit cannot be mounted in exactly the same position with respect to the sprocket assembly after each time it has been removed, the adjustment process becomes a burdensome trial-and-error process.
Third, because these sprocket assemblies involve multiple movable parts, determining how to distribute the forces among the various parts of the sprocket assembly has been difficult. Since the generally V-shaped swinging sector pivots about a hinged side relative to the stationary sector, the most displacement of the swinging sector occurs at the periphery of the sprocket assembly. If the load is to be taken by the stationary sector at the parting side, as compared to the hinged side, the periphery of the swinging sector at the parting side would have to be at least as thick as most extreme position of the periphery of the stationary sector. Of course, any additional thickness of the sprocket assembly adds weight to the bicycle and makes the sprocket assembly more difficult to manufacture.
In addition, because the foregoing sprocket assemblies are shifted by a means other than a derailleur, there is a need for a device capable of maintaining the tension in the chain when the chain is shifted between the sprockets. There is also a need for a device capable of maintaining as much of the chain as possible entrained around the sprocket at all times.
Attempts have previously been made to keep the chain tensioned and entrained around the sprocket assembly by coupling a flexible rod, similar to a fishing rod, at one end to the frame of the bicycle and rotatably coupling a jockey wheel to the opposite end to engage the lower side of the chain loop and force the chain up into engagement with the bottom portion of the sprocket. This arrangement proved unsuccessful, however, because the rod would bounce excessively under normal operating conditions which would create slack in the chain and cause the chain to separate from the sprocket.